This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-260269, filed Sep. 14, 1999; and No. 2000-098024, filed Mar.31, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a constant current circuit using a current mirror circuit. More specifically, the invention relates to a source-type constant current driver circuit and a sink-type constant current driver circuit which are used to drive an LED (Light Emitting Diode) etc.
Conventionally, as for the constant current circuit using the current mirror circuit, the source-type constant current driver circuit for driving the LED, for example, as shown in FIG. 1, is known. Here, description regarding circuits will be made assuming that the number of output bits is xe2x80x988.xe2x80x99 In FIG. 1, for example, a reference voltage source 101 for supplying a reference voltage is connected to one of input ports of an amplifier (Amp.) 102. An output port of the amplifier 102 is connected to a base of an NPN transistor (multiplying factor: xc3x9710) Q101 for generating a reference current. An emitter of the transistor Q101 is connected to the other input port of the amplifier 102 and also connected to a terminal (REXT) 103. The terminal 103 is connected to one end of an external resistance R used for control of the output currents. Other end of the resistance R is connected to a terminal (GND) 104.
A collector of the transistor Q101 is connected to a base of a PNP transistor (xc3x9710) Q102. Moreover, the collector of the transistor Q101 is also connected to a collector of one transistor Q103a of a PNP transistor (xc3x9750, xc3x9750) pair Q103a, Q103b. By the way, the transistor pair Q103a, Q103b is configured to have a current ratio of 1:1 and constitutes a current mirror circuit 105. Moreover, a collector of the transistor Q102 is grounded and an emitter thereof is connected to a common node of bases of the transistor pair Q103a, Q103b. Further, a collector of the transistor Q103b is connected to a collector of an NPN transistor (xc3x9710) Q104 and a base of an NPN transistor (xc3x975) Q105, respectively.
On the other hand, a terminal 106 for supplying a power-supply voltage VDD (for example, 5V) is connected to each emitter of the transistor pair Q103a, Q103b and also connected to a collector of the transistor Q105.
An emitter of the transistor Q104 is connected to the terminal 104 and each emitter of NPN transistors (xc3x9710, . . . ) Q106a to Q106h, respectively. A base of the transistor Q104 is connected to an emitter of the transistor Q105. Moreover, a base of the transistor Q104 is connected to each base of the transistors Q106a to Q106h through switches 107a to 107h, respectively.
Here, the transistors Q106a to Q106h are provided according to the number of the output bits (in this case, 1 to 8 bits). In addition, each of the transistors Q106a to Q106h together with the transistor Q104 constitute the current mirror circuit whose current ratio is set to 1:1, respectively.
Each collector of the transistors Q106a to Q106h is connected to a base of a PNP transistor (xc3x9710) Q107, respectively. Moreover, each collector of the transistors Q106a to Q106h is connected to a collector of one transistor Q108a of a PNP transistor (xc3x9750, xc3x9750) pair Q108a, Q108b. By the way, the transistor pair Q108a, Q108b constitutes a current mirror circuit 108 whose current ratio is set to 1:1. Here, for convenience"" sake, here only the circuit for a first bit of the output is shown in the figure.
A collector of the transistor Q107 is grounded, and an emitter thereof is connected to a common node of bases of the transistor pair Q108a, Q108b. Further, the collector of the transistor Q108b is connected to a common node of bases of an NPN transistor (xc3x9710, xc3x97150) pair Q109a, Q109b and also connected to a collector of one transistor Q109a of the transistor pair Q109a, Q109b. Moreover, the transistor pair Q109a, Q109b is configured to have a current ratio of 1:15 and constitutes a current mirror circuit 109.
Furthermore, each emitter of the transistor pair Q108a, Q108b and a collector of the transistor Q109b are connected to a terminal 110 for supplying a power-supply voltage VCC (for example, 17V), respectively. In addition, each emitter of the transistor pair Q109a, Q109b are both connected to a terminal (Out) 111.
According to the source-type constant current driver circuit of such a configuration as this can yield heavy-current outputs (in this case, 160 mA) each of which is formed by multiplying a reference current (for example 10 mA) by a factor of n according to an amplifying factor (current ratio) of the current mirror circuit 109. However, in the conventional source-type constant current driver circuit mentioned above, a useless circuit current (consumption current) i that reaches as high as 1/12 to 1/20 or so of the output current flows. Especially when the number of the output bits is large, the power consumption of the circuit increases because of the circuit current i according to the following formula: power consumption of the circuit=VCC voltagexc3x97output current/ratio of the circuit current ixc3x97the number of bits. Therefore, the circuit has a demerit that circuits having a large number of bits of the output are not suitable for a small size package.
For example, now, assume that a heavy-current of 160 mA is outputted and the circuit current i of 1/16 times the output current flows uselessly. Then, a power consumption of the circuit is obtained, according to the formula, as power consumption of the circuit =17Vxc3x97160 mA/16xc3x978=1.36 W. Further, a fact that the useless circuit current i is large means that desired output characteristic cannot be achieved unless the transistors are designed to be in large sizes. Therefore, the circuit of this type tends to bring about a larger chip size and an largely increased cost.
As described above, although the conventional base voltage control type of source-type constant current driver circuit makes possible a stable output of constant current, the useless circuit current at the time of outputting a heavy-current is large. Therefore, the circuit has problems that its power consumption tends to become large and it is liable to have a larger chip size and a largely increased cost.
The object of the present invention is to provide a constant current circuit capable of decreasing its power consumption and also capable of being manufactured in a smaller size at the same time with a reduced cost.
A constant current circuit according to one aspect of the present invention, comprises a first transistor for generating a reference current in conformity to a reference voltage, a generator circuit for generating a current of 1/xcex2 times the reference current that is supplied for a base current of the first transistor, a transistor pair for amplifying by a factor of n the current that is generated by the generator circuit so as to be 1/xcex2 times the reference current, and a second transistor to which a current that is an n times amplified current by the transistor pair is supplied for a base current thereof.
According to the constant current circuit of the present invention, even when a heavy-current is required, the useless circuit current consumed by the circuit can be reduced to a small amount. As a result, a constant current circuit can be constructed as a low-consumption constant current circuit comprising small-size transistors.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.